What Do You Know About Microbes? (pre-assessment)

Can You Estimate the Mass of One...?

This portion of the activity is designed to focus students' attention on the large quantity of microbes normally found inside and on a healthy person's body. To establish a point of comparison, have students examine a paper clip and then estimate its mass (weight). Explain that the mass (weight) of one paper clip is 1 gram.

This might be a good time to explain the difference between mass and weight, since the terms are often used interchangeably. In classical physics, mass is a measure of an object's resistance to changing its state of motion when a force is applied. It corresponds conceptually to "how much matter" is in an object. Weight is a measure of the effect of gravity (such as the Earth's gravitational field) on an object. Technically, a beam balance measures mass, whereas a spring scale (which examines how much gravity pulls on an object) measures weight. The distinction between mass and weight is not important in everyday circumstances, because the strength of gravity hardly varies from location to location on Earth, making weight uniformly proportional to mass.

If time allows, you might want to have students measure other common items. For instance, a penny has a mass of 3 grams. Next, tell students to take turns holding the 1-liter water bottle to estimate its mass. Have students record their estimates on a small sticky notes. Direct students to create a class graph by placing their estimate notes on the board in ascending order, with estimates of the same value stacked above each other in vertical columns (see illustration above).

Ask students to examine the class bar graph. While analyzing the information on the graph and discussing students' estimates, you could introduce the measures of central tendency (mean, median, mode and outliers). Inform students that the actual mass of the water-filled bottle is 1,000 gm. Ask, Was anyone's estimate close? Explain that the mass of microbes found in and on a person's body also is approximately 1,000 grams!